Are you on Windows? If so you can adjust firewalls, like Windows Firewall, to block all data to any specific application, allowing your other apps to remain online.
Start->Control Panel->Windows Firewall->(upper left) Allow a program or feature through Windows Firewall. Scroll the list, find Flash/Builder and allow or disallow.
Otherwise it's pretty much an on/off button with the internet. Rather than potentially moving hardware and plugging and unplugging the net cable, especially being notorious for being fragile, I'd go in Start->Control Panel->Network and Sharing Center->Local Area Connection->Disable. Or Wifi if you use that.
Careful though. The nature of Windows (since its black and white inception) has been to hang applications that require networking, especially when an application had internet when it was started. Microsoft still hasn't figured out how to use async threads yet. It's like a parallel port printer..
